14 



PRODUCTIVE FARMING 



several kinds polliwogs, water snails, and small fish. Set 

 the aquarium in a very light place. When the plants begin 

 to grow, they will give off oxygen to the water. The animals 

 will breathe the oxygen from the water. A little careful 

 observation will tell whether there are 

 too many animals in the aquarium or 

 not. If the fishes come to the surface 

 to breathe a great deal, some of them 

 should be taken out. Snails and polli- 

 wogs are scavengers and will eat the 

 waste matter gathering on the inside of 

 the glass and on the plants. Do not 

 let the pupils feed the fishes too much. 

 This will spoil the water for the fishes. 

 How Plants Get Water. The 

 moisture from the soil is the only water 

 taken in by farm crops. This enters 

 through the thin covering of the root- 

 hairs growing on the fibrous roots. 

 The absorbing surface is very great and 

 large quantities of liquid are taken 

 up. It is estimated that several hun- 

 dred tons of water or liquid food are taken up by a corn crop 

 to produce one ton of dry fodder. 



Osmosis is the name of the process by which liquid food 

 is taken into the plant through the roots. A little of the acid 

 or other cell sap in the roots passes into the soil at the same 

 time. There is a sort of trading of two liquids through the 

 surface membrane of the root-hairs. A very little of the 

 denser liquid of the plant is sent out in exchange for a large 

 quantity of soil water. 



Osmosis may be defined as the exchange of two different 

 liquids through a thin membrane which separates them. 

 Two liquids separated by a membrane will pass through it 

 and mingle with each other. Soil water will thus reach and 



FIG. 5. A sunlight 

 aquarium. The plants fur- 

 nish oyxgen ana the ani- 

 mals use it. The animals 

 produce carbon dioxide 

 and the plants use it. 



